Recently much effort has been made to obtain energy from sources that previously were considered unfeasible or uneconomical. Such sources include biomass, refuse, and slurry-type fuels. One specific example is the spent shale which is a product of processing oil shale. Spent shale can be defined as including the characteristics of a low BTU, low volatile content, low carbon, high ash fuel. Spent shale typically comprises a wide range of particle sizes, e.g. from fines to about 3 inches. While such spent shale contains considerable useful energy, known combustion systems have not been able to efficiently utilize this energy because of the relatively wide range of particle sizes. In known entrained or fluidized bed combustors, fine size particles; i.e., less than 1 inch, are effectively handled but such combustors are inadequate for relatively larger particles. In other types of combustors, coarse size particles; i.e., about 1-4 inches, are combusted, but such a system cannot tolerate any significant amount of smaller size particles or fines due to the unacceptably high pressure drop created by such particles.
In order to obtain useful energy from the spent shale, it has been necessary to first separate the spent shale particles into fine and coarse size particles before introducing the separated coarse particles into a combustor adapted to handle such particles and separately introducing the fine particles into another combustor adapted to handle these smaller particles. Alternatively, it has been suggested to subject the larger particles to a grinding operation to form smaller size particles. Because the spent shale is generated at a relatively high temperature; e.g., about 425.degree. C., the grinding step causes a substantial removal of heat from the spent shale by contact with the grinding equipment. Since it is highly desirable to maintain the spent shale at this high temperature, the grinding operation results in a wasteful loss of heat.
One method for burning pulverized fuel such as pit coal is disclosed in U.S. Pat. No. 4,475,472. This method involves using a modified fluidized bed furnace such that a primary air stream supplies the pulverized fuel above the fluidized bed in the furnace. The fuel is then separated above the fluidized bed with the finer particles being burnt in the flame above the bed, and the coarse particles falling into the bed itself for combustion. The fuel is supplied to the fluidized bed furnace via a primary air pipe which extends through the fluidized bed and opens into the flame area of the burner.